The present invention relates in general to solid-state lasers and pertains, more particularly, to a microwave-driven (MWD) UV solid-state laser. The laser of this invention provides an efficient, compact, and tunable solid-state laser and is an improvement over the conventional microwave-driven solid-state laser.
The present invention further relates in general to a UV light source, and more particularly to a microwave driven excimer light lamp.
With conventional solid-state lasers it is generally necessary to provide a broadband source, such as high-pressure, and high-current rare gas flashlamps for pumping solid-state lasers. This approach can be traced to the initial demonstration of an operable ruby laser in 1960.
In theory, broadband sources should radiate the majority of their energy in the visible to near infra-red. However, the plasma that is produced is optically thick for visible to infra-red transmissions due in part to self-absorption and Doppler and pressure broadening. Other drawbacks are well known for the conventional broad-band sources and their various applications.
Narrow-band sources are known as drivers for solid-state lasers. One application of a narrow-band source is the use of a frequency doubled Nd:YAG laser or a dye laser as an excitation source to drive a tunable Ti:Al.sub.2 O.sub.3 laser. Another example of the use of a narrow-band source are the 50% to 60% efficient narrow-band semi-conductor lasers which emit at approximately 810 nm, coupled with fiber optics or coupled to the side of a Nd:YAG rod.
Another drawback associated with the narrow-band semiconductor lasers is an overall efficiency of converting heat to 810 nm light of only 20%, approximately. Although this is considered relatively efficient, the single semi-conductor lasers will be likely to produce more than 100 milliwatts of continuous power or less.
Therefore, a solid-state laser such as Nd:YAG in which the coupling of the 810 nm light is assumed to be 40% efficient, and which has a 50% efficiency of converting absorbed fluorescence into laser photons, would require at a minimum an array of fifty (50) semi-conductor lasers to produce each watt of solid-state laser output.
Conventional UV sources, for use as a light source and for a laser and other applications are also inefficient and therefore costly to operate. Further, conventional light sources do not include waveguides, and are typically limited to strictly UV ranges.
It is expected that high average power, narrow band photon sources, if available, would have a number of commercial applications. High average power, narrow band photon sources can be categorized as either lamps (incoherent) or lasers (coherent). Lamps can be either continuum or line and can produce large quantities of light with good efficiency and are relatively inexpensive.
One drawback is that lamps produce substantially lower intensities than lasers and are generally an overall less efficient device due to poor geometric coupling due to the incoherent quality of the light and, more particularly, because of their broadband emission spectra. Another drawback is the potentially detrimental heating from the lamp due to the light produced at wavelengths other than those required by a specific commercial process at the absorption band of the commercial process to which the lamp is applied.
It is known to select and use laser sources, for example, diode, dye, and other solid-state lasers, to closely match an absorption band of a specific commercial process. Thus, the photons are used efficiently, heating is minimized, and geometric coupling is accomplished more efficiently since the laser output is coherent.
Lasers have a number of drawbacks due to the generally accepted expense of lasers, their known inefficiencies (possibly excluding diode lasers from this category), and their inability to produce large amounts of average power. It will be recognize, therefore, that lasers are not currently usable for any number of commercial applications.
Thus, if efficient, narrow band lamps could be made available, then they would provide a relatively inexpensive source of photons for commercial lamp applications. Conventional lamps are recognized to have a number of drawbacks. Existing conventional lamps have substantial problems with efficiency and broad band emission.
All known existing lamps are variations of the two basic types of lamps and generally operate as either electrical discharge lamps or hot wire thermal radiators. Hot wire thermal radiators produce a "black body" continuum. The "black body" continuum is determined by the temperature of the wire.
Electrical discharge lamps typically utilize rare gasses or metal vapors, for example, Hg or Na. Electrical discharge lamps produce line or continuum spectra which depends on pressure, temperature, and discharge power density. Electrical discharge lamps are inherently inefficient for commercial processes with narrow absorption bands because of the limited overlap between emission and absorption spectra.
It is believed that much higher efficiencies could be obtained if a lamp's emission could be limited to the absorption band or bands of the specific commercial process. Unfortunately, another drawback associated with known line radiator lamps is the relatively low power and/or inefficiency. For example, if the transition is between excited states, i.e., rare gas line radiators, the lamp is inefficient because the excitation energy is much greater that the photon energy.
If the transition is to a ground state as is the case for Hg and Na lamps, then the lamp can be efficient because the-photon energy can be a significant fraction of the excitation energy. However, these lamps are limited to low pressures, small sizes, and low excitation power densities. The lamps are thereby limited to low output power.
Another drawback with these lamps is that at high pressures or for large volumes, the high optical density of the lamp causes the emitted radiation to be self-absorbed. The self-absorbed radiation will be re-emitted but with the loss of the line nature of the emission. This is due to significant broadening of the line as is observed for high pressure Na and Hg lamps.
If high power is attempted from a small volume with a line radiator-type lamp, then the high temperatures, pressures, and electron densities produced broaden the line or lines. The line or lines broaden until the lamp becomes a continuum radiator. This is the case for high power rare gas .discharge lamps.